Create a Certificate parameter
    
    Up until now, Bytes parameter was used for certificate parameters
    throughout the framework. However, the Bytes parameter does nothing
    special for certificates, like validation, so this had to be done
    for each of the parameters which were supposed to represent a
    certificate.
    
    This commit introduces a special Certificate parameter which takes
    care of certificate validation so this does not have to be done
    separately. It also makes sure that the certificates represented by
    this parameter are always converted to DER format so that we can work
    with them in a unified manner throughout the framework.
    
    This commit also makes it possible to pass bytes directly during
    instantiation of the Certificate parameter and they are still
    represented correctly after their conversion in the _convert_scalar()
    method.
    
    https://pagure.io/freeipa/issue/4985
    
    Reviewed-By: Fraser Tweedale <ftweedal@redhat.com>
    Reviewed-By: Rob Crittenden <rcritten@redhat.com>
    Reviewed-By: Martin Basti <mbasti@redhat.com>